1. Field of the Invention
The invention relates to new hydrophobic polyols of low viscosity, to a process for preparing them and to solvent-free binder mixtures based thereon which are especially suitable for primers of floor coatings.
2. Description of the Prior Art
Prior-art solvent-free two-component (2K) coating systems divide essentially into epoxy resin (2K EP) systems and polyurethane (2K PU) systems.
Coatings based on 2K EP systems combine good mechanical strength with high resistance to solvents and chemicals. Additionally they are distinguished by very good substrate adhesion. A distinct disadvantage is their poor elasticity, particularly at low temperatures. This brittleness results in poor crack bridging by the coating, so that an attack may take place here on the substrate. An additional disadvantage is the very low resistance to organic acids. This is a problem above all for applications in the food sector, since in that sector organic acids are often released as waste products.
A balanced combination of hardness and elasticity, on the other hand, is the outstanding property of the 2K PU coatings and the greatest advantage over 2K EP coatings. With similar solvent and chemical resistances, moreover, the resistance to organic acids of 2K PU coatings is substantially better than that of 2K EP coatings.
For environmental reasons coating compositions ought to be solvent free, particularly in the case of high-build applications, such as floor coatings, for example. This means that the inherent viscosity of the binder component ought to be low.
In the case of high-build applications on the basis of 2K PU systems there is a risk of bubbles forming through the formation of CO2 as a consequence of the water-isocyanate reaction. Therefore it is important that the raw materials exhibit very low water absorption, so that such coatings can be applied without bubbles even under damp conditions. Since the hydroxy-functional component is generally more hydrophilic than the polyisocyanate component, it is particularly important to use hydroxy-functional components that are hydrophobic, specifically.
The hydroxy-functional binder component of the 2K PU coating may be constructed on the basis of a variety of types of chemical structure.
Whereas polyester polyols are distinguished by a low viscosity and relatively low water absorption, their stability to hydrolysis is low, thereby severely restricting their usefulness for the corrosion protection of metallic substrates and also for the coating of mineral (alkaline) substrates.
2K PU coatings based on polyacrylate polyols are distinguished by effective resistance to hydrolysis, but a disadvantage is their relatively high viscosity. Accordingly either solvents or reactive diluents such as polyether polyols or polyfunctional alcohols are always added to adjust the viscosity. In the same way as when polyether polyols are used alone as crosslinkers, this generally has the effect of increasing the water absorption behaviour.
Solvent-free polyols are often made sufficiently hydrophobic in the prior art by, using castor oil (e.g. Saunders, Frisch; Polyurethanes, Chemistry and Technology, Part 1 Chemistry pages 48 to 53). The 2K PU coatings produced therewith, however, are not stable to hydrolysis.
Resins derived from cashew nut shell liquid (CNSL; cashew nut shell oil) are used for paints and coatings (Shukia et al., in Paintindia, February 2002, pages 29-32, Nayak, Natural Oil-Based Polymers: Opportunities and Challenges, in J.M.S.—Rev. Macromol. Chem. Phys., C40(1), 12-18, 2000). Such products are very hydrophobic and because of the absence of ester bonds they are stable to hydrolysis. CNSL-based resins contain phenolic OH groups, which can be utilized for crosslinking by reaction with polyisocyanates. A disadvantage here, however, is that the phenol-based urethane bonds formed are not stable to re-cleavage under damp, alkaline conditions.
J48-29530 describes the reaction of CNSL-aldehyde or cardanol-aldehyde condensates with alkylene oxides such as propylene oxide, the phenolic OH groups becoming aliphatically attached hydroxyl groups. A disadvantage of these products is the fact that the viscosity is relatively high and that the addition of solvents is necessary for processability.
An object of the present invention was therefore to provide a hydrophobic, low-viscosity polyol component which can be further processed to solvent-free binder mixtures and which does not exhibit the aforementioned disadvantages of lack of resistance of the coatings based thereon, especially in the field of high-build applications.